mb/google/rex/var/screebo: Refactor SSD power sequencing

Improve SSD readiness time by enabling earlier power sequencing.

Here are the two GPIOs to look for:
* GPP_A19: Power Enable
* GPP_A20: PERST

The flow is presented as `stage (GPIO PAD/Value)` for easy
understanding:

bootblock (A20/0, A19/1)
|
v
romstage (A20/1)
|
v
ramstage (A19/1, A20/1)

Ideally, we don't need SSD power sequencing at ramstage, but due to the
fact that Screebo has RO locked, any change in the bootblock won't be
applicable for FSI'ed screebo devices. Therefore, we're keeping the
existing ramstage power sequencing flow as is.

TEST=Able to build and boot google/screebo using NVMe without any
problems. S0ix and read/write from/to SSD are also normal.

Change-Id: I0ee1fa4613178da8771c9e6b5ee871e50ea6324c
Signed-off-by: Subrata Banik <subratabanik@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/80640
Reviewed-by: Kapil Porwal <kapilporwal@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

0 files changed, 0 insertions, 0 deletions